The present invention relates to a synchronization acquisition scheme for radio terminals of a radio communication network.
In the description below, the radio communication network may be a second generation mobile communication network as IS-95 or GSM as well as, a third generation cellular mobile radio communication networks based on Code Division Multiple Access (CDMA) as UMTS, or any other radio communication network where a radio terminal has to acquire synchronization before setting up a communication with the network.
The problem of acquisition can be seen as an attempt to synchronize in time the radio terminal clock to that of the base station.
In the following description will be assumed that a radio terminal is attempting synchronization for the first time before starting any communication mechanism. This procedure is known as slot synchronization. This invention concerns not restrictively the slot synchronization, it can be extended to frame synchronization or another similar application.
Within a cellular mobile radio communication network, a radio terminal, when switched on, must find among several surrounding base stations, the base station which provides the strongest signal power and acquire its synchronization before beginning a communication. Each base station of a cellular mobile radio communication network is responsible for including synchronization sequences in the transmitted signal flow so that a radio terminal is able to acquire, from the received signal flow, the synchronization of the most appropriate base station. FIG. 1 shows the transmit part of a base station describing how the synchronization sequence is included in a transmitted signal flow. An IQ signal modulation and spread spectrum are illustrated in this example, the invention is however not restricted to this framework.
The transmit part of the base station comprises a multiplexer 11, scramblers 12, adders 13, pulse filters 14, a local oscillator 15, modulators 16. Multiplexer 11 multiplexes several channels CH1, . . . , CHn. The output of multiplexer 11 is duplicated and transmitted on two parallel paths called I and Q. On path I as well as on path Q, the output of multiplexer 11 is connected to a scrambler 12, added a predefined quantity at adder 13, and submitted to pulse filter 14. The output of pulse filter 14 is modulated at modulator 16 with a signal delivered from local oscillator 15 on path I, and from a signal derived from oscillator 15 and shifted by pi/2 on path Q. The output of said modulators 16 on path I and on path Q are added and transmitted on the radio channel. The predefined quantity Cp is periodically added at adder 13 on path I building a slot structure.
FIG. 2 represents a time axis showing the slot structure supported by both path I and path Q. Each time slot on path I has a duration of TS and comprises the predefined synchronization sequence Cp, called synchronization sequence, at the same position in each time slot. Preferably the synchronization sequence is at the beginning of the time slot and has a duration SYNC shorter than the whole slot duration. For example in current UMTS standard the time slot duration is equal to 2560 times the chip duration while the synchronization sequence duration is equal to 256 times the chip duration. Each time slot on path Q comprises a predefined quantity Csi. Assumed that a frame comprises M time slots, M quantities Csi 1<=i<=M are defined and successively included at the same position in each one of the M time slots constituting the frame. Preferably, the predefined quantities Csi are located at the beginning of the time slot on path Q and have the same duration SYNC as the synchronization sequence Cp.
The predefined synchronization sequence Cp may be a Gold sequence of length 256.
Alternatively, the use as predefined synchronization sequence Cp of “A new correlation sequence for the primary synchronization code with good correlation properties and low detector complexity,” Tdoc SMG2 UMTS L1 427/98 introduces the concept of hierarchical sequences. Such synchronization sequences have computation repetition properties within the correlation. Correlation computation using hierarchical sequences, introduces intermediate values which can be frequently reused along the correlation procedure. The synchronization sequence Cp may also be the one described in “Generalized hierarchical Golay sequence for PSC with low complexity correlation using pruned efficient Golay correlators” 3G TS 25.101 v3.1.0 (1999-12). The above mentioned synchronization sequences are only cited for purpose of illustration, the present invention does not depend on the used synchronization sequence.
The initial synchronization procedure at the radio terminal comprises usually the following three steps:
Step 1: Slot (chip) synchronization. The radio terminal uses I path to get slot (chip) synchronization to the strongest base station.
Step 2: Frame synchronization and sub-group identification. This step allows to determine the position of the slot detected in Step 1 within the frame. For this purpose, the radio terminal uses path Q.
Step 3: Base station identification. The radio terminal finally determines the scrambling code that identifies the strongest base station.
Slot synchronization at the radio terminal is achieved by using a filter matched to the predefined synchronization sequence Cp repeatedly emitted by each base station, on a slot-by-slot basis. In fact, for each part of the signal flow received at the radio terminal and having a duration of a time slot, the synchronization sequence is present inside this part of signal flow. As the radio terminal does not have any a priori knowledge of the exact position of the synchronization sequence, the radio terminal has to perform several correlations to detect the beginning of the synchronization sequence. Assumed that a time slot contains N chips and that the sampling factor is equal to 1, a correlation profile P is preferably composed of the result of N correlations P(1), . . . , P(N) of a part of the signal as large as the synchronization sequence with the synchronization sequence itself. The part of the signal considered is shifted by a chip duration or, more generally speaking, by a sample of it, for each successive correlation.
The result P(k) of the correlation between the part of the signal flow shifted by k chip durations (assuming a sampling factor of 1) and the predefined synchronization sequence Cp is given by equation:       P    ⁡          (      k      )        =            ∑              i        =        0                    n        -        1              ⁢                  Cp        ⁡                  (          i          )                    ·              r        ⁡                  (                      i            +            k                    )                    where r(t) is the received signal flow at time t, k=1,2, . . . , N and n equals the size of sequence Cp.
If the sampling factor is higher than 1, for example 2 or 4, the number or correlations to be performed to generate a correlation profile is preferably equal to N*sampling factor.
The correlation profile reveals correlation peaks for each base station located in the surroundings of the radio terminal. Detection of the position of the strongest peak gives the timing of the base station providing the strongest signal power at the radio terminal. The synchronization acquisition at the radio terminal further consists in comparing the strongest peak with a threshold value.
Known solutions consist in:                accumulating peaks over a variable number of slots and comparing the strongest peak with a fixed threshold value, or        non-coherently accumulating peaks over a fixed number of slots and comparing the strongest peak with a variable threshold value.        
These two solutions lead to an important computing time and energy consumption at the radio terminal.
The method shown in WO-9731428 involves generating a number of user channels, a side channel and a null channel by a base station. Each channel uses a different pn code and the channels are applied to a transmitter. A number of subscribers receive the signals, despread the signals using their assigned pn codes and measure the received signal level. The obtained received signal level is used when the phase of a second pn code is set which corresponds to the side-channel to achieve synchronization.
This document doesn't disclose a method for optimization of the number of time slots needed to achieve synchronization.
The method shown in EP-852430 involves receiving a control signal in the control channel of a base station, a part of the control signal in every long code period is spread only by the specific short code, and other parts of the control signal are spread by one of the synthesized spread code sequence. The control signal is correlated with the specific short code to produce a correlation signal. A long code synchronization timing of the control signal is determined based on the correlation signal. The correlation between the control signal and segments of the synthesized spread code sequences is detected, the synthesized spread code sequences are synthesized from the different long codes and the specific short code. Each segment is taken from a portion of a respective one of the synthesized spread code sequences, starting from a first position in a first of the synthesized spread code sequences from where a first segment is taken, and in successive synthesized spread code sequences, starting from a position in each successive synthesized spread code sequence which is shifted by a set shifting amount from where a preceding segment was taken from a preceding one of the synthesized spread code sequences. The set shifting amount is less than a length of each segment, and the control signal and each segment are synchronized by the long code synchronization timing. The different long codes synthesizes the one of the synthesized spread code sequences is identified, which corresponds to the control signal, based on the electric power level of the correlation signal.
This prior art document doesn't discloses means for optimization of the process.
The method shown in U.S. Pat. No. 5,570,349 involves measuring the time base error of each handset at the base station. Time base correction information is transmitted from the base station to each handset. The timing of operation of each handset is thus controlled.
The power of the sounding signal is measured. The compensation of the handset transmit power using instantaneous power control of handset to base station link is performed so as to avoid signal interference.
The major drawback of these solutions relies in the way of determining the number of slots to get reliable synchronization. When the number of slots is too long, an unnecessary amount of computations is performed. Conversely, when the number of slots is too short, the probability of acquiring wrong synchronization will be increased. In such a case, the mobile terminal will perform the procedures that follow synchronization with low reliability so that the need for restarting the whole synchronization mechanism will be very likely. Accumulating the matched filter outputs over a large number of slots does not guarantee a reliable estimation of the strongest base station.